DENIERS SAY:

Some scientists believe in climate change, and some don't. It's a topic still up for debate.

SCIENCE SAYS:

Climate change is real. Just ask 97% of the top climate scientists or any national science academy in the world.

There is overwhelming evidence that our climate is warming due to pollution from human activities. That's the conclusion reached by 97% of the top climate scientists and every major National Academy of Science in the world. When we burn dirty fossil fuels like oil and coal, and when we cut down forests that store carbon, we pollute our atmosphere and warm our planet. This is not controversial: It's a reality we've understood for decades.
Now here’s an experiment: Ask a good scientist what is known about a particular topic, and watch how quickly that scientist moves to what is unknown. Honesty about what is unknown is essential to sound science, and no entire field of science can ever be completely settled. But the basics of climate change ARE settled. In other words, there’s no dispute among the vast majority of climate scientists that our climate is warming, and that most of the recent warming is due to human activity.

Additional info from Skeptical Science

Skeptics often claim that the science of anthropogenic global warming (AGW) is not “settled." But to the extent that this statement is true it is trivial, and to the extent that it is important it is false. No science is ever “settled”; science deals in probabilities, not certainties. When the probability of something approaches 100%, then we can regard the science, colloquially, as “settled."

The skeptics say that results must be double-checked and uncertainties must be narrowed before any action should be taken. This sounds reasonable enough — but by the time scientific results are offered up to policymakers, they have already been checked and double-checked and quintuple-checked.

Scientists have been predicting AGW, with increasing confidence, for decades (indeed, the idea was first proposed in 1896). By the 1970s, the scientific community was becoming concerned that human activity was changing the climate, but was divided on whether this would cause a net warming or cooling. As science learned more about the climate system, a consensus gradually emerged. Many different lines of inquiry all converged on the 2007 conclusion by the Intergovernmental Panel on Climate Change (IPCC) that it is more than 90% certain that anthropogenic greenhouse gases are causing most of the observed global warming.

Some aspects of the science of AGW are known with near 100% certainty. The greenhouse effect itself is as established a phenomenon as any: It was discovered in the 1820s and the basic physics was essentially understood by the 1950s. There is no reasonable doubt that the global climate is warming. And there is also a clear trail of evidence leading to the conclusion that it’s caused by our greenhouse gas emissions.

What about the remaining uncertainties? Shouldn’t we wait for 100% certainty before taking action? Outside of logic and mathematics, we do not live in a world of certainties. Science comes to tentative conclusions based on the balance of evidence. The more independent lines of evidence are found to support a scientific theory, the closer it is likely to be to the truth. The fact that some details are still not well understood should not cast into doubt our understanding of the big picture: Humans are causing global warming.

In most aspects of our lives, we think it rational to make decisions based on incomplete information. We will take out insurance when there is even a slight probability that we will need it. Why should our planet’s climate be any different?